In the past, wireless systems employed fixed antenna systems with antenna patterns that were carefully engineered to achieve desired coverage characteristics. In these systems, as in most wireless systems, the link between a base station and a portable unit is for communicating data in the uplink and downlink directions. Uplink is when the portable unit is the transmitter and the base station is the receiver, and downlink is when the base station is the transmitter and the portable unit is the receiver. Although these fixed antenna systems are effective for less sophisticated wireless communications, they are not effective for today's wireless systems. Today's wireless systems must be capable of handling dynamic changes in data traffic requirements.
Smart antennas offer a broad range of ways to improve the performance of today's wireless system. Some of these improvements relate to enhanced coverage, reduced infrastructure costs and increased system capacity. System coverage is improved by increasing the gain of the base station antenna without increasing the output power of base station or subscriber units. Smart antenna technology can also be used to improve various types of wireless systems such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). FDMA and TDMA systems are improved by having increased carrier-to-interference ratios, which allows for more frequent reuse of frequency channels. CDMA systems are improved because subscribers transmit less power for each link, which reduces multiple access interference and increases the number of simultaneous subscribers that can be supported in a single cell.
Smart antennas use a fixed set of antenna elements in an array. The signals from these antenna elements are combined to form a movable beam pattern that can be steered, using either digital signal processing, or radio frequency (RF) hardware, to desired direction that tracks mobile units as they move. This allows smart antenna systems to focus RF signals on a particular subscriber, while minimizing the impact of noise, interference, and other effects that can degrade signal quality.
Smart antennas can also be used to spatially separate signals, allowing different subscribers to share the same spectral resources, provided that they are spatially separable at the base station. This is known as Space Division Multiple Access SDMA, which allows multiple users to operate in the same cell, and on the same frequency or time slot. Since this approach allows more users to be supported within a limited spectrum allocation, compared with conventional antennas, SDMA can lead to improved capacity.
In general, smart antenna systems use beamforming techniques that combine a number of data signals from a number of low gain antenna elements. Beamforming can be realized in various stages of a wireless system, such as baseband, IF, or RF. A baseband beamforming is normally performed digitally, while the RF beamforming is done in analog mode. IF beamforming can be carried out in either digital or analog mode. Between the two, the digital beamforming is more cost-effective, accurate and reliable. However, using IF beamforming it is more challenging to achieve sufficiently high processing speed and resolution in uplink and downlink directions of a wireless system.
Thus, it is desirable to provide an apparatus, system, method and computer program product for improved IF digitial beamforming in a wireless network. The apparatus, system, method and computer program product of the present invention disclosed herein address this need.